1. Field of Invention
The present invention relates, in general, to a powertrain for a hybrid vehicle and, more particularly, to a powertrain for a hybrid vehicle which can reduce the number of gear pairs, used to form gear stages, by removing an engine clutch from a transmission and by combining a planetary gear unit with the gear pairs, thereby reducing the production cost and weight of the transmission, and which can directly downshift the transmission into a target lower gear when performing kickdown, so the powertrain can realize improved gear shifting performance.
2. Description of Related Art
Parallel hybrid systems for hybrid electric vehicles (HEV) are typically classified into FMED (Flywheel-Mounted Electric Device) systems and TMED (Transmission-Mounted Electric Device) systems according to locations of motors in the systems.
FIG. 1 shows a conventional TMED hybrid system, in which a motor is connected to a transmission and which can perform an EV mode (Electric Vehicle mode) in which the system drives a vehicle using motor power exclusively.
In the EV mode, the TMED hybrid system drives the vehicle using only the motor power when the vehicle is started or runs at a low speed. In the EV mode, an engine clutch is disengaged, and only the rotating force of the motor is transmitted to the wheels, so the vehicle is driven by the motor power exclusively.
In addition, in an HEV (Hybrid Electric Vehicle) mode, the system drives the vehicle using both the engine power and the motor power. Here, when a driver changes the mode from the EV mode to the HEV mode while driving the vehicle in the EV mode, a high level of shock may be generated at the moment the engine power is coupled to the motor power. To prevent the generation of shock when changing the mode, the system synchronizes the engine rpm with the motor rpm after the engine is started, and engages the engine clutch so that the motor and the engine can be efficiently coupled together.
However, the conventional hybrid powertrain having the above-mentioned construction is problematic in that the engine clutch is added to between the engine and the motor, thereby causing a problem in packaging the hybrid powertrain components and increasing the production cost of the transmission.
Further, for a vehicle equipped with an AMT (Automated Manual Transmission) or a DCT (Dual-Clutch Transmission), both transmissions having manual gearbox mechanisms, external gear pairs are provided to realize respective gear stages, thereby increasing the production cost and weight of the transmission and causing a problem in packaging the hybrid powertrain components. Furthermore, when performing kickdown, gears are sequentially shifted (for example, fourth gear→third gear→second gear, third gear→second gear→first gear), thereby causing a delay in gear shifting response and reducing the performance of gear shifting response.
In addition, although the vehicle having the hybrid powertrain is operated in the EV mode when the vehicle is started or runs at a low speed, the period the vehicle runs in the EV mode is short, thereby limiting an increase in fuel efficiency of the vehicle.
The information disclosed in this Background section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art.